rtio: introduce await operation

Introduce await operation which simply blocks execution until the
SQE is signaled using the new API rtio_sqe_signal(). This is useful
for locking the bus for synchronous operations, especially those
requiring precise timing. For example, performing a transaction
within a tiny and exact time window.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>